Thermionic device and means and method of fabrication



March 20, 1945 W. W; WATROUS, JR

THERMIONIC DEVICE AND MEANS AND METHOD OF FABRICATION Filed Oct. 17, 1942 2 Sheets-Sheet 1.

ll-llll INVENTOR mm mmeaua rz WWW I ATTORNEY March 20, 1945. w, w, wAi'Rous, JR

THERIIIONIC DEVICE AND MEANS AND METHOD OF FABRICATION Filad' 0a.. 17. 1942 2 Sheets-Sheet 2 INVENTOR mmlrgrrzwgwe ATTORNEY Patented Mar. 20, 1945 r Q'FFlCE THERMIONIC DEVICE MEANS AND METHOD OF FABRICATION wwiw. Watrous, Jr. comm, N. 1.1, assumia WestinghwseElcctric 8: Manufacturing Com- Pa-ny, East Pittshur Pennsylvania H gh, Pa... a corporation of Apn ioati'on mm 11, 1942,. was... 4: 2.342. comma (01.250. 47.51, This. invention relates tothermioniddevices andmeans and method of fabrication thereof,

and isdirected more particularly to the cathode structure and its manufacture-and assembly, and has relation more especially to cathodes capable of employing high potentials.

The use of high potential, that-is, any potion-- tial above approximately ten volts, for energizing the cathode of a. gas or vapor filled electronic tube. has been extremely limited due tothe dimculties. involved in the construction and use of such a device. As a matter of 'fact, despite the desirability of high cathode potentials for cathode supply, probably 95% of the gas or vapor filled tubes used today operate with cathode potentials under seven volts.

The major difficulty in the use at higher cathode potentials is the fact that as soon as anyportion of the cathode, which has attained an emitting temperature, assumes a potential in excess of approximately twelve volts t'instantaneous) in respect to any other: portion of the cathode, the gas or vapor content: of the tube mentioned the fact that the. insulation 01 that.

character readily cracks due to expansion and thermal shock. Such cracks may expose the heater or filament so that discharge can occur or in severe cases so that an actual short-circuit occurs. Furthermore, insulation becomes lessinsulative and more conductive astemperatures are increased, and this tendency to become conductive has rendered insulation ot'the character described undesirable but heretofore necessary inthe character of tubes'of present concern. Impurities in the insulative material will greatly exaggerate the condition lust mentioned of conductivity, and the expense and difiiculty: oi; purification, oi the material to be used as the insulation also. is an adverse condition to use oi indirectly heated cathodes by insulated filaments,

An object. of the invention accordingly is to avoid the drawbacks of a heavily coated heater filament.

An equally important: object of the invention is to provide acathode which will be capable of operation at highvoltages withoutdetrimental eiiects arising-= afiecting tube life or, operation.

Another object of the invention is to provide a cathode which will avoid arcing with a cathode potential in ex'cessof present-day usual voltage ior' heater purposes of indirectly heated cathodes. A iurther obiectof the invention'istoprovide a cathode of the character indicated which is of practical manufacture and assembly.

A still further object offlthe invention is to provide a cathode of the characterindicated utilizing -a heater within the same which is. prevented from arcing, to the cathode in use with high potentials. 1

Another object of the. invention is'to enable a, cathodev heater to" be situated in vacuum in a gas or vapor filled tube;

Yet another objectof the invention is toVprovide an improved method oi fabrication of a tubeiulfillingathe above objects-in whole or in Again, an object of the invention is to provide apparatus. for. evacuating the cathode-heaterregion: and seal the same against. ingress: of th gas or vapor of the. discharge tube.

. Additional objects of the invention will appear as the description progresses, both bv'direot recitationthereof and by implication from'the con,-

Referring to the accompanying drawings in which like numerals or reference indicate. parts throughout the several views;

Figure l a vertical sectional view of a thermionlc device wherein my improved cathode is shown. in longitudinal section; v

Figure 2; is a cross-sectional view on the line II'--II of Fig. 1;; a

Figure 3 is: a vertical elevational view of" a cathode and associated parts viewed as indicated by" arrows III--III'of Fig. I, and showing a modified configuration of cathode;

Figure 4 is a. cross-sectional view on the.- line net-Iv of Fig. 1; v

Figure 5 is a sectional elevation. of apparatus for evacuation of. the cathode-heater region, and showing" a cathode andv associated partssituated therein. for evacuation; and,

Figure-61s a. modified construction or cathode lnaivinov tribulation connection showntherewith tor'purposes of evacuation.

provide anlindirectlyheated cathode which will o In specific embodiment of the invention illustrated in said drawings, the reference numeral l designates a glass or other envelope having a base ll thereon and a stem l2 therein as customary in the art. In the particular showing in the drawings, the stem is illustrated as having a collar clamp 13 thereon from which extend upwardly supporting rods M in turn mounting a grid I5 of foraminous material in the region above the stem and within the envelope. An anode I6 is depicted at the upper part of the envelope within a cylindrical extension ll of the grid. g

A cathode in accordance with the present invention is situated coaxially within the grid and has support from the stem. "Said cathode is preferably a hollow metallic shell providing a cylindrical wall l8 the upper end of which is closed by an end wall I9 shown as integrally,

formed with the said cylindrical wall with the outer surface of both said walls coated with suitable electron emissive coating or otherwise rendered electron emissive. Within the shell, coaxial therewith, is an insulator 2| of generally cylindrical shape and longer than the shell. The end wall IQ, of the shell has an appropriate pocket 22 pressed therefrom to receive and position the end of said insulator 2|.

At the lower end of shell. 18 of the cathode is a transverse disc-shaped metallic plate 23 which has a central orifice somewhat larger than the diameter of the aforementioned insulator 21 but of less diameter than the said shell. At the part of said platerunderlying the end of said shell, said plate is impressed with an annular trough 24, of ample capacity for receiving the said end of the shell and a continuous filling of solder 25 bywhich a vacuum-tight seal is obtained between the shell and plate. Use of plate 23 promotes inductive heating formelting the solder during fabrication and constitutes a shield be-. tween the stem and lead-in wires and the main part of the interior of the-envelope where ionizationtakes place .in use of .the device. Projecting from below and into the orifice of said plate and sealed to said plate by welding or otherwise, is a sleeve .26 constituting amountingmeans for the cathode. The lower part of said mount-. ing. meansor sleeve 26 shoulders outward and.

then downward to. provide an integral enlarge-' mentZT the lowerrim whereof is sealed in the glass of stem l2. Preferably said sleeve is .an integral piece of material of cylindrical shape having a coefficient of expansion which is sub-. stantially thatof the glass of the press. .Material sold inthe trade under the trade-name of Kovar may well be used for the purpose. Withinthe said sleeve 2.6 and engirdling a lower end portion of said insulator 2i which dependsinto the sleeve substantially the full length thereof, is an intervening collar of insulative material 28 which according positions the insulatorcoaxially with respect to the sleeve, plate and shell.

A heater filament 29 such as a coiled spiral shown, is wound around the insulator 2| for the length thereof from the. upper end of collar 28 substantially to the upper end of. the insulator but sufiiciently below the upperend so as to be out of contact from the end wall of the shell. Theinsulator is shown as having a central bore therethrough thereby enabling an upper end connection for the heater filament to be extended downwardly therethrough and to protrude into the region confined by the enlargement of the sleeve. and its seal withthe stem. Theotherend of the filament coil has a connection which extends downwardly into this same region by passing between the insulator and said collar 28. The filament may be coated with a thin layer of insulation if desired as a precaution against short circuiting between parts thereof which nections are secured to appropriate lead-in wires 38, 30 which are sealed through the stem in usual manner. The regions within the shell N3 of the cathode and between the sleeve and the stem are intercommunicating due to the looseness of the collar and insulator and of the filament wire connection within each, or otherwise as found desirable as by communicating holes or grooves, so that evacuation of the one region obtains evacuation of both. It will be observed that said regions are wholly confined so that a vacuum may be maintained therein. Such a vacuum is provided, and in consequence thereof it is now to be observed that the heater filament is operating in a vacuum irrespective of what pressure. of gas or vapor may exist on the ex-v terior of the cathode within the envelope.

. The ;particular device shown is one wherein the envelope is evacuated to remove the air therefrom and thereafter'has a gas of desired character and quantity admitted into the envelope before sealing, The completed device consequently comprises a tube containing a gaseous atmosphere and having therein an evacuated enclosure for a heater which is thus operating in vacuum while the surrounding part of the tubeis operating with gas present. The heater voltage therefore will have no opportunity to cause gas breakdown even though that voltage "-1 is, considerably higher than ordinarily used in present-day practice. The structure accordingly enables metoaccomplish the desired result of operating the filament heater at considerably more than seven volts, so that I am' enabled to connectthe filament direct to commercial lines and operate the device with 110, 220 or other voltages if so desired. 1

The method of fabrication by which I obtain a sealed enclosure for the cathode heater filament is illustrated. in one embodiment in the showing particularly of Figures 1 and 5. Apparatus used in conjunction with the method will be simultaneously described. Before assembly of the stem and cathode in the envelope, evacuation of the heater region is efiected. For that purpose, the stem is mounted temporarily in a housing 3| which in the present showing comprises a glass dome 32 having an exhaust connection 33 at its top. The bottom edge of the dome is sealed to a copper or othersleeve 34 the lower margin of which is in turn soldered or otherwise secured vacuum tight, to a rigid collar 35 having an'internal' diameter sufflcient to admit insertion of the stem and cathode therethrough into the lnterior of the dome. The outside of the collar is threaded at 36 to receive a threaded coupling ring 31. The collar 35 and coupling ring 36 have inwardlydirected flanges 38, 39 respectively adaptedzto tightly clamp therebetween a rubber orotherwasherfifl superposed upon an annular lit at an inwardly directed insulative, nipple 4a io-rmed with a tapered inner end 43 constitutin atemporary seat for the lower end of, the stem i2. Said nipple also has a. deep pocket 44 for reception of the tubulation 45 forming partof the stem structure and 'has spring clips 45 on its sides to receive and grip the filament lead-in wires Suitable electrical connectors 41 are provided from the clipsto the bottom of the nipple enabling heating current to be applied to' the filament while mounted in the apparatus being described. It is now appropriate to observe that the'appa-ratus thus assembled is'a complete enclosure for the stem and cathode and may be.

subjected to evacuation which thereby effects evacuation of the interior region of the-cathode shell, prior to the soldering of said shell to plate 23.

Upon introduction of the stem andcathode into i the evacuating apparatus of Fig, 5, shell 18 is not yet sealed to plate 23, but isloose with respect thereto thereby enabling evacuation within the dome to be effective to evacuate air from within the shell. A ring of solder issituated around the base of the shell ready for melting, and when out-gasing and evacuation has been accom plished, augmented by running the filament at a high temperature, a high frequency coil 48 placed around the dome obtains an inductive heating of the plate 23 sufiicient to melt the ring of solder which thereupon flows into the trough NWhere it is allowed to harden and which then seals the shell vacuum tight in place. There being no other openings into the interior region of the cathode, it is thereafter an evacuated region and may be removed from the evacuating apparatus and the tube completed as shown in Fig. 1.

A .structure of cathode capable of evacuation through tubulation' is illustrated in Figure 6. Furthermore in this modified construction, evacuation is not extended to include the region between the cathode and stem, but is confined to the region within the cathode only. Describing this modification more in detail, stem l2a is provided with an upper cylindrical neck 49 around which is mounted the lower end of a metallic cylindrical sleeve 50 having appropriate fit there-, on for the purpose. The upper part of said sleeve projects above the neck portion of the stem and overlaps the lower end of a cylindrical cathode 'shell l8a to which it is welded or otherwise secured. The cathode shell in this instance is shown as tubular and of course electron emissive on its outer surface, and is closed at both ends by headers or end walls of which the upper one is identified as upper end wall Na and the lower one is identified as bottom end wall 5|. Each is shown as fitting within the cathode shell and as,

having an outwardly directed flange 52 which fits the shell and is welded or otherwise sealed thereto. The upper end wall has a pocket 22a pressed upwardly therefrom and the bottom end wall has a similar pocket 53 pressed downwardly therefrom, both pockets being coaxial with the shell and adapted to receive the ends of a cylindrical insulator 21a supporting and positioning the same. A filament 29a is shown as wound spirally around the insulator, the upper end of the spiral being connected through a lateral slot 54 next the upper end of the insulator to make electrical connection downwardly through a central bore of the insulator and outwardly through another lateral slot 55 to a lead-in wire 56. The lower end of the spiral part of the filament makes connection to another lead-in wire 51. Said leadmay be in the range, say from 50 to 5000 microns.

provides means for evacuation of the interior region of: the cathode shell. by virtue of, the passage madeby the presence-v of. slot Minthe insulatnr .ot the aforementioned opening ea in; the upper endwall, Tubulation 60 is, preferablymetal and after evacuation has been. accomplished, tubulation. is pinched and sealed together closeh; above the said upper wall and tubulation severed at; or above the pinched, sealed portion- The interior region is accordingly readily evacuated and. maintained in the evacuated conditi0n.s0 that the assembly can be completed by addition'of the grid, as in Fig. 1, to the stem and the stem sealed within the envelope.

While in the foregoing description cathode shell l8 and la hasbeen defined as cylindrical, it is to be understood that other shapes and configurations of hollow cathode may be employed of which an additional example i given in Fig. 3. In that figure the cathode shell l8b is shown as having a corrugated wall. which constitutes one illustration not only of a. different. configurationof cathode, but also of a cathodeof limited length having increased emissive surface. Except for this difference in. Fig. 3, this viewmay be considered an elevation of the cathodeand stem assembly otherwise shown in Fig. 1, its lower end being sealed in trough 24 of plate 23 as in that figure. I f

While in the foregoing description of the sev'-'- eral' constructions shown reference has been made to the evacuation and maintenance of the interior region of the cathode evacuated, it is within the scope of the invention to refill said region with a suitable gas at a relatively high pressure as compared to the pressure in the body of the envelope. The pressure difference should of course ,be well above a pressure at which breakdown or end-to-en discharge would occur,

and as example thereof, the pressure difference should be in the range, say from 25 to 760 mm.

Where the cathode is evacuated the pressure difference thereof from the body of the envelope Examples of gases that are contemplated are xenon, krypton, neon, argon, helium, nitrogen,

hydrogen and the like, which will not combine with materials present in or forming the cathode. Presence of gas within the cathode region when of this suitable character, aids in'heat transfer from the heater to the cathode shell and will thus enable the heater to operate at alower temperature. The invention accordingly is exemplified in either construction, namely, a radical difference of pressure is. maintained between the interior of the cathode and the interior of the body of the envelope. The tube will therefore operate in the part or the. voltage-pressure curve well above the low dip of the curve and in the parts of the curve tapering toward infinity.

Since the various details of construction, as well as the precise relation and functioning of parts, are subject to variation and change without departing from the inventive concept or scope of the invention, it is intended that all matter contained in the specification or illustrated in the drawings, shall be interpretedas exemplary and not in a limiting sense. It is also. to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein shown and described and all statements of the scope of the invention herein set forth as a matter of language which might be said to fall therebetween. 1 1 1 I claim: L 1. A cathode comprising a hollow shell having an exterior emitting surface, a heater within said shell, means closing the upper end of said shell to external pressure, means closing the bottom end of the shell to external pressure, and means rigidly carried by the respective closing means at both the upper and bottom ends of said shell, centering said heater coaxially therein, said shell maintaining pressure withinthe same different than pressure outside said cathode. 2.- A cathode comprising a hollow shell, an upper end wall for'said shelLsaid end wall having a centrally disposed pocket therein; an insulator longitudinally situated in said shell and projecting at its upper end into said pocket, and means at the lower endof said insulator positioning said insulator coaxially of the cathode and sealing the cathode to maintain a vacu therein.

3. A cathode and stem assembly comprising a glass stem having a sleeve sealed thereto, a plate transverse to and sealed to said sleeve, said plate having an annular trough therein, a closed cathode shell having one end thereof sealed in said trough, and a heater within said shell, said stem, sleeve and shell providing a confined region and said region being evacuated thereby preventing ionization and discharge between the heater and cathode.

4. A cathode comprising a hollow shell, an end wall at each end of said shell sealed thereto, each end wall having a pocket, an insulator longitudinally situated in said shell and carried at its ends in saidv pockets, and a tubulation sealed to one of said pockets, said pocket to which said tubulation is sealed having an opening from the interior of the shell into said tubulation for evac-' uationpurposes, said hollow shell being evacuated and said tubulation pinched and sealed closed for maintaining the vacuum in said shell, and a heater on said insulator within said shell and operating in the vacuum in said shell.

5. A method of fabrication of a hollow cathode for mounting said cathode on a stem and evacuating the cathode, comprising positioning a cathode in a trough carried by the stem, providing solder in position to flow around said cathode in said trough, placing a dome over the cathode and stem temporarily sealing the dome and evacuating the same, and melting said solder and causing it to seal the cathode in said trough for maintaining the acuum within said cathode and the cathode assembled on the stem.

WARD W. WATROUS, JR. 

